The following relates to the electric power arts, energy conservation arts, electric power grid management arts, and related arts.
In electric power grid management, power flow and frequency balancing is performed at various time scales.
Primary frequency control, sometimes called “frequency response”, is performed on a time scale of seconds or fractions of a second, and provides a “first response” mechanism to arrest a frequency disturbance due to an unexpected event such as a power generator abruptly going offline. Primary frequency control is a local process in which generators or loads adjust their power based on the locally detected electrical frequency. Generally, if the electrical frequency is above the target frequency then the generator produces less power, while if the frequency is below the target frequency then the generator produces more power. By way of illustration, FIG. 5 shows a typical generator droop control curve for a generator providing primary frequency control. In this example, the generator speed is locked with the electrical frequency (hence the ordinate being labeled “Frequency/Speed”), but as the system frequency moves up or down, the reference speed of the generator is adjusted (via throttling fuel or steam, for example) to move the power output to match. This allows multiple synchronous generators on the system to instantly respond to changes in system frequency and share the load.
Secondary frequency control, sometimes call “frequency regulation”, operates on a time scale of a few minutes or less, and provides balance to maintain a desired frequency, e.g. 60 Hz in North America. Secondary frequency control is usually performed by a Supervisory Control and Data Acquisition (SCADA) system in accordance with an Automatic Generation Control (AGC) signal. Secondary frequency control entails substantial communications infrastructure, and has higher latency time as compared with primary frequency control. Moreover, some implementations of secondary frequency control may not be fully automated.
Frequency control on longer time scales is usually referred to as tertiary or time control, and is performed on the basis of a “clock” defined by the time-integrated frequency. If the “clock” is running slow (frequency too low) then the target frequency may be increased slightly (e.g. by a few millihertz) to compensate, while if the “clock” is running fast then the target frequency may be decreased slightly.